This application relates generally to a chemical process and, more particularly, to a process for synthesizing a thiacyanine dye.
British Pat. No. 1,445,870, issued Dec. 8, 1976, discloses the incorporation of a particular class of thiacyanine dyes into a silver halide photosensitive emulsion in order to enhance the sensitivity of the emulsion to the blue region of the visible spectrum. The preferred thiacyanine dye is anhydro-5-chloro-3'-ethyl-3-(4-sulfobutyl)-thiacyanine hydroxide ##STR1##
This compound may be prepared by adding 2-ethylthio-3-ethyl benzothiazolium tosylate to a warm solution of 5-chloro-2-methyl-3-(4-sulfobutyl)-benzothiazolium betaine, adding triethylamine and stirring vigorously. The desired product can be recovered from the reaction mixture by filtration, purified and recrystallized. This reaction is not completely satisfactory because ethyl mercaptan is formed as a by product. Ethyl mercaptan is a noxious chemical which boils at 35.degree. C. and the complete elimination of this material from the product is extremely difficult. Accordingly, it would be desirable to have a synthesis for such thiacyanine dyes which would not form an alkyl mercaptan as a by-product. The present invention is drawn to such a synthesis.